The present invention relates to a hammer drill. More particularly this invention concerns a chuck and tool for a hammer drill.
In a hammer drill it is necessary to secure the shank of a drill bit in the workpiece holder or chuck. The tool is axially reciprocated and rotated, so that it is necessary to allow at least limited axial displacement of the tool in the chuck, yet be able to transmit rotary motion between the chuck and the tool.
Normally the tool is simply formed with a recess that is closed at both axial ends and of part-cylindrical shape. The chuck has a normally cylindrical receptacle adapted to receive this shank and is provided with a radially inwardly engageable tool-securing element that is movable into a position in the recess. Thus this element serves not only to prevent the tool from falling axially out of the chuck, but also serves to transmit force angularly between the chuck and the tool.
A considerable disadvantage with this system is that the tool-holding element and the corresponding recess are subjected to considerable force so that they tend to wear, with the recess enlarging and the element becoming smaller. After some use, particularly when the tool is employed to drill holes in masonry and the like, the fit between the tool and the chuck becomes so very loose that it is necessary to use a new tool and/or replace the holding element.
It has been attempted to overcome this disadvantage by replacing the normally part-spherical end of the element which engages in the recess with an axially elongated cylindrical member so that a greater contact surface is obtained. Such an arrangement does increase the service-life of the combination somewhat but nonetheless leaves this element and the recess as the most wear-prone parts of the combination.